Condition and spatial distribution
The condition of individual bush-crickets
influence their spatial distribution
DAG Ø. HJERMANN
Department of Biology, Division of Zoology, PO Box 1050 Blindern, N-0316 Oslo, Norway
Abstract
Males of many species of grasshoppers and crickets (Orthoptera) form aggregations when calling for
females. However, to our knowledge it has not been reported that the males’ spatial position within
aggregation correlates with mating success, such as in many species of lekking birds and mammals.
Males of the wart-biter Decticus verrucivorus, a bush-cricket (Tettiginiidae), are attracted by other
males’ song and so have an innate tendency to aggregate. They also provide no parental care. Their
mating system therefore resembles a lek (but not a "classical" lek, since males hold floating
territories).
In this experiment, I "created" animals of good and poor condition by rearing
approximately half of the animals under suboptimal conditions. The animals were released in equal
density in eight experimental patches of three sizes (20, 40 and 80 m2), and subsequently tracked by
recording their positions four times per day during three weeks. Males that were raised under
suboptimal conditions moved less, sang less and had lower song amplitude than the other animals,
confirming that they were of poorer condition. The movement patterns revealed that in medium-sized
patches, poor-condition animals of both sexes stayed more in the edge zone (< 1 m from habitat edge)
than expected by chance, while good-condition animals used the edge zone and interior equally much.
The same tendency was found in the only small patch with good-condition animals during most of the
experiment. During the peak hours of male singing, however, poor-conditioned females (but not
poor-conditioned males) moved towards the center of the patch, where predominantly good-condition
males were advertising. In the large patches, however, animals used the interior and the periphery
equally much regardless of condition. The results indicate that in the small and medium patches,
poor-condition males are repelled by males of better condition because they latter sing louder or more
frequently. The spatial segretation between good- and poor-condition females was surprising; the
most plausible explanation appears to be that females compete for access to attractive males. Wartbiter males may only mate once a day under ideal conditions, and more infrequently under Norwegian
climatic conditions. The lack of differences in the edge-interior use in the large patches may be
caused by larger habitat area (less need or higher cost of defending the interior) or by larger
population size (the optimal behavioral strategy depends on population size). The interaction between
condition and patch size is interesting in the light of the excessive habitat fragmentation of many
habitats, including the wart-biters’ natural habitat.
1
Paper III
Introduction
In many species of grasshoppers and crickets (Orthoptera), the males form
aggregations when calling for females (e.g., Campbell and Clark 1971; Schatral 1984). In
some species, males seem to clump mainly because the preferred microhabitat, usually tall
plants, is clumped, i.e., there is no need to invoke an innate tendency to clump to explain the
observed distribution (Arak and Eiriksson 1992). In these species, clumping may reduce the
ability of males to attract females (Arak et al. 1990), so when the area of suitable microhabitat
is limited, males may be forced to settle more densely than optimal. In contrast, males of the
wart-biter Decticus verrucivorus are attracted to other singing males (postive phonotaxis;
Schatral et al. 1985; Keuper et al. 1986). This, together with the impression that wart-biter
males congregate in smaller areas than the preferred vegetation, lead Weidemann et al. (1990)
to propose that the wart-biter mating system is a resource-based lek, i.e., that positive
phonotaxis leads to a denser aggregation than one would expect from the distribution of
microhabitat alone. The male does not provide parental care (Wedell and Arak 1989; Wedell
1993), so it indeed appears likely that wart-biters actually display a lek in the relative broad
sense ("an aggregated male display that females attend primarily for the purpose of mating";
Höglund and Alatalo 1996:6).
In many species of lekking birds and mammals, "dominant" males tend to stay in the
middle of the male aggregation (Tab. 3.1 in Höglund & Alatalo 1995). However, this has to
my knowledge never been reported in Orthoptera. In the current study, I studied the spatial
movement patterns of wart-biters that were manipulated so they were of either good or poor
condition. The animals were individually marked and their movements were followed during
an extended period of time in experimental patches of different sizes. I specifically tested
whether "dominant" males tended to stay in the middle of the male aggregation, and whether
the spatial orginazations of the animals was affected by habitat or population size.
Methods
Study species and experimental design
The wart-biter (Decticus verrucivorus L.) is a relatively large bush-cricket (length 34 cm); it has wings, but most individuals are not capable of flying more than a couple of
meters (Ander 1947). In Norway, adult males are largely found in July and August. Adult
males emerge before females, probably to avoid sperm competition by mating with virgin
2
Condition and spatial distribution
females (Wedell 1992). The diet of the wart-biter is a combination of insects (especially
acridid grasshoppers) and plant food. Both instars and adults are extremely heat-loving, and
the habitat (in Scandinavia) is low meadow or grassland vegetation, preferably including
microhabitats with tall vegetation (male singing perches) and vegetation-free spots (where
females prefer lay their eggs; Ingrisch and Boekholt 1982; Cherrill and Brown 1990; Cherrill
et al. 1991). Adults sing mostly between 1000 and 1200, on hot days also around 1500.
Males move frequently around during these periods, singing for a few minutes from each
perch.
Females approach the male when they want to mate; mating is very short (1-2
minutes).
The experiment was performed during the summer of 1997 at Oslo University’s
experimental station at Evenstad, SE Norway. Eight habitat patches were made by mowing
parts of a continuous area of seminatural pasture. Because the available area was shaped as a
narrow rectangle, all habitat islands were 4-6 m wide, while their lengths varied from 4 to 16
m. Two islands were relatively large (ca. 80 m2), two were medium (ca. 40 m2), and four
were small (ca. 20 m2) (Fig. 1). Since wart-biters prefer a habitat mosaic of open ground and
vegetation (Cherrill and Brown 1990), I created a number of vegetation-free spots within each
patch. The patches were protected from bird predation using nets suspended 1.5 - 2 m above
the ground (the wart-biters could move freely through it). To the north and south, adjacent
patches were separated by 20-25 metres of short-cut lawn. To the western side there was 80100 cm of earth, then a 60-cm fence of transparent plastic seaparating the study area from a
corn field. On the eastern side, there was ca. 2 m short-cut lawn between the patch and a 60
cm high fence of solid metal. There were no wart-biters on the study site in advance. The
animals were caught in semi-natural meadows and roadsides in five locations in SE Norway.
Some were caught as 2.-3. instars and reared in the laboratory until they were adult, while
Wire fence
Corn field
Habitat patches
Bare ground
Metal fence
10 m
50 m North
Figure 1. Map of the experimental area. The experimental patches are arranged in a row between a
corn field in the west and another experimental area in the east. Between the patches was extremely
short-cut grass.
3
Paper III
others were caught as adults shortly before the experiment. In the laboratory, the insects were
kept individually in plastic boxes (measuring 16 x 8 x 5.5 cm) with a bit dry vegetation (to
climb on to prior to moulting) and fed with cabbage leaves and ground dried catfood. Partly
because there was not enough space for the final moult, the laboratory conditions were clearly
suboptimal, as evidenced by small physical deformities and lower song performance and
movement activity in males (see Results). I therefore refer to laboratory-reared and adultcaugh animals as the groups "poor condition" and "good condition", repectively. The animals
were marked by gluing one end of a red, numbered 80 x 4 mm plastic tape to the top of the
pronotum, cooled down and released in the experimental habitat patches at about 5 p.m. to
avoid escape reactions because of handling. On June 24 at 0500, adults were released in equal
densities in each of the eight patches (4, 8 and 16 in the small, medium and large ones,
respectively) in approximately even sex ratios (male:female number was 5:3 in one medium
patch and 9:7 in the large ones) and with approximately even proportions of good- and poorcondition animals. However, 23 of the animals migrated between patches during the study,
especially from the small patches; migration rate was especially high in good-condition males.
The populations in the small patches were therefore biased towards poor-conditioned females
at the end of the study. Animals were usually tracked four times a day on almost every day
from day 4 to day 23 of the experiment and on two days thereafter. In each tracking session, I
measured the position of each individual relative to a 25 x 25 cm grid which was marked in
the patches using small poles. During tracking, the animals were usually not disturbed so
much that they moved. I also noted activities such as singing, mating and egg-laying. For
each male, I assessed the song volume on a scale from 0 (not audible) to 3 (very loud).
Temperatures were recorded (usually 20 - 30 times/day) using an ordinary ethanol
thermometer lying sun-exposed on ground level leaves in order to mimic the body
temperatures of basking wart-biters.
Movement activity
Movement activity or speed was analysed by linear regression, based on the distance
between two subsequent recordings on the same day (r) and the time interval (t) between the
two recordings. The transformation loge(r2) was used as the response variable, while the
predictor variables were ln(t) together with covariates (these transformations are suitable to
describe random walks and similar movement patterns; Johnson et al. 199x, Hjermann 2000).
I used the following covariates: the identity number of the animal (ind) and the mean
temperature during the time interval between recordings (temp, calculated from temperature
4
Condition and spatial distribution
measurements smoothened out using 2nd degree polynomial regression), and temp2. I tested
whether any of the covariates could be excluded from the model using Mallow’s Cp. On the
individual level I tested the effect of patch size, sex, catch location (the geographic origin of
the animals), and condition (lab-reared vs. wild-caught) on movement activity, using each
individual’s estimate of the ind parameter as an indicator of movement activity.
Use of the edge zone vs. the patch interior
As a condensed measure of choice of microhabitat within each patch, I calculated the
use of edge zones vs. the patch interior for each individual. The edge zone of the patch was
defined as the part of the patch lying within 1 m of the patch edge. Because the patches were
relatively small, I assumed the entire patch to be equally available to the animals. I defined
the selection index SI following Manly (1974, eq. 3): SIedge = ratioedge/(ratioedge + ratiointerior),
where ratio = (proportion of use)/(proportion of area). SIedge was calculated for each
patch*animal combination with at least five observations.
SIedge can vary from 0 to 1.
Animals outside the patch were not included when SIedge was calculated.
A rough measure of the range area of each animal was calculated from the median as
well as the 10th and 90th percentiles of the x- and y-coordinates (xmed, x10, x90, etc.). The range
area was calculated as the area of the irregularly diamond-shaped area between the points (x10,
ymed),(xmed, y90), (x90, ymed),(xmed, y10). I did this for all animal-patch combinations with at least
eight observations.
Results
Movement and song activity
The animals moved quite much on sunny days, but some of them could spend
several days within a limited area (Fig. 2). The optimal model (as measured by Mallow'
s Cp )
to explain movement length between subsequent observations on the same day (r) included all
the variables considered: the time between observations (t), the average temperature during
that time interval (temp), temp2, and the identity of the individual (ind):
log(r2) = 1.047 log(t) + 0.72 temp - 0.0109 temp2 + ind + ε
5
Paper III
All factors were significant with P < 0.001. The random component ε had an
estimated variance of 3.82 and appeared normally distributed. In the case of a random walk,
the coefficient of log(t) is theoretically expected to equal one (Johnson 1992); thus, the value
of 1.047 (SE: 0.147) indicates that the wart-biters’ movement paths were close to a random
walk on this scale. The temp and temp2 coefficients indicate that the movement length
increased with the temperature (measured in the sun) for temperatures up to 28-30° C and
flattened out thereafter. Lastly, the significance of the ind parameter (P < 0.0001) indicates
A4 (lab-reared male)
A5 (lab-reared female)
22 A ug
8 A ug b
1 Sept
8 A ug
6 A ug
Mark f ound
28 July
31 July
patch on 18 A ug
1 A ug
4 A ug a
5 A ug
13 m f rom the
28 July
4 A ugb
4 A ug
27 July
6 A ug
30 July 12 A ug
SIe dge = 0.53
5
7 A ug
14 A ug
SIe dge = 0.70
A2 (adult-caught male)
A3 (adult-caught female)
1 Sept
8 A ug
8 A uga
22 A ug
7 A uga
31 July
3 A ug
8 A ug
27 July
4 A ug
27 July
15 A ug
7 A ugb
14 A ugb
5 A ug
13 A ug
SIe dge = 0.58
12 A ug
12 A ug
30 July
14 A uga
SIe dge = 0.49
Fig. 2. Four representative examples (one for each "type" of animal) of animal’s movements within a
medium patch (4x8 m). Some dates are marked to give an impression of the temporal scale. The border
between the edge zone and the interior is marked with a broken line. The edge selection index SIedge for
each individual is note expected to be 0.5 when the edge zone and the interior is used equally much.
(Note that this index does not reflect the area use of the A5 female very well, partly because
observations outside the patch are ignored.)
6
Condition and spatial distribution
P oor-condition m ales
0,7
Good-condition m ales
E gg-laying, fem ales
0,6
0,14
0,5
0,1
0,4
0,08
0,3
0,06
0,2
0,04
0,1
0,02
0
Proportion e gg-laying
Proportion singing
0,12
0
8
10
12
14
16
18
20
Hour of da y
Fig. 3. Proportion of observations where males sang (squares, on the left axis) and where females laid
eggs (crosses, right axis) during the day.
that movement activity differed between individuals. Movement activity, as indicated by each
individual’s estimate of ind, did not depend on patch, patch size, catch location or sex
(ANOVA, P > 0.40).
There was an interaction between sex and condition (F1,50 = 4.76,
P = 0.034): males caught as adults (males in good condition) were significantly more active
than males caught as instars and reared in the lab (males in poor condition; t21 = 5.68, P <
0.001).
The same tendency prevailed for females but was not statistically significant
(t29 = 2.10, P = 0.16).
As expected, singing activity was highest in from ca. 10 a.m. to about noon and
declined thereafter (Fig. 3). In addition to display higher movement activity, males in good
condition also tended to sing 3-4 times more often than males in poor condition (song
frequency per individual; t21 = 5.89, P < 0.0001; Fig. 3). When they sang, good-condition
males tended to sing much less loud than poor-condition males (means 1.08 vs. 2.79 on a
scale from 0 to 3; t21 = 4.00, P < 0.001). Egg-laying was observed only 22 times (mostly just
outside the patches); it was most frequently observed in the afternoon and evening (Fig. 3).
There was no difference in egg-laying frequency with regard to condition (Fisher’s exact twosided test, P = 0.64).
7
Paper III
Table 1. Tests of effects of sex, rearing (lab-bred or adult-caught) and patch size on the use of edgezones, measured by the overall habitat use and by habitat selection within days. Single factors were
tested using one-way anova, while the tests for interaction effects are type III tests from two-way anovas
(tests of the main effects are not shown).
Overall habitat use
Selection within days
Effect
d.f.
F
P
d.f.
F
P
Sex
1,59
0.01
0.97
1,53
0.01
0.91
Sex x Rearing
1,57
0.03
0.87
1,51
0.25
0.62
Sex x Patch size
2,55
1.41
0.25
2,49
1.12
0.33
Patch size x Rearing
2,55
3.59
0.034
2, 49
3.84
0.028
Patch size (among lab-reared animals)
2,40
6.36
0.004
2, 37
6.68
0.003
Patch size (among animals caught as adults)
2,15
0.76
0.49
2, 12
0.97
0.41
Use of edge zone vs. patch interior
Use of the edge zone as measured by SIedge did not differ among the sexes, but
differed among patches for animals reared in the lab. In the medium-sized patches, poorcondition animals stayed significantly more in the edge zone than in the interior (selection
indices > 0.5, Fig. 4, Tab. 1). In the small and large patches, the selection indices indicate that
Ed g e se le ctio n in d e x
poor-condition animals on average used edges and interiors equally much. This effect of
Poor condition
0,7
G ood condition
0,5
0,3
0,5
S m all
1,5 M edium 2,5
Large
3,5
Patch size
Fig. 4. Average edge selection index for poor- and good-condition animals on patches of three sizes.
The indicated standard errors (1SE) were calculated using each individual (not each wart-biter
observation) as one statistical observation.
8
Condition and spatial distribution
#%$&$('*),+-$/.%0/1 231 $(.546(798/: 6<;
= $-$&0/)3+-$/.%0(1 231 $/.54>6/798(: 6-;
#%$&$('*),+-$/.%0/1 231 $(.798/: 6-;
= $-$&0/)3+-$/.%0(1 231 $/.798(: 6-;
"
!
)?A@-@&@
C@-@&@B)
- @
& @B)
%%@-@
%%@-@B)
Fig. 5. Each individual’s average distance to the patch center during the day, based on the medium patches
only. One standard error is indicated; standard errors were calculated using each wart-biter observation as
one statistical observation.
patch size was not found among good-condition males, and the interaction between patch size
and condition was significant (Tab. 1). However, because most good-condition animals left
the small patches early in the experiment, the estimates of good-condition animals in these
patches are based on only four animals, of which two were were male immigrants who stayed
for a short time. In the only small patch were the two groups coexisted for an extended
period, the two good-condition animals (one male, one female) stayed more in the patch
interior (SIedge = 0.32 and 0.37) than the three poor-condition animals (SIedge = 0.45, 0.62 and
0.69). Thus, this patch shows the same tendency as the medium patches: poor-condition
animals of both sexes used the edge more than good-condition animals.
To further explore the social processes in the medium patches, I calculated the
distance of each observation to the patch center for different times of the day. I found that
although the overall habitat use of the sexes is quite similar, poor-condition females approach
the center of the patch in the period between 1000 and 1230, in the period when male singing
is most intense (Fig. 5). Thus, females of both poor and good condition were spatially
associated with adult-caught males in this period.
As expected, range areas were smallest in the small patches, but ranges were not
larger in the large patches relative to the medium-sized ones (Fig. 6). Range areas did not
9
Paper III
Males
16
F emales
Males
Are a (sq. m )
12
8
4
0
0,5
S m all
1,5
M edium
2,5
Large
3,5
P a tch siz e
Fig. 6. Area of the wart-biter ranges, calculated as explained in the text. There were no significant effect
of neither sex nor the condition treatment, but a significant effect of patch area (F2,53 = 6.26, P=0.004). The
indicated standard errors were calculated using each individual (not each wart-biter observation) as one
statistical observation.
differ between the sexes or between lab-reared and wild-caught animals, nor were there any
significant interactions.
As expected, range areas were smallest in the small patches, but ranges were not
larger in the large patches relative to the medium-sized ones (Fig. 6). Range areas did not
differ between the sexes or between lab-reared and wild-caught animals, nor were there any
significant interactions.
Discussion
The results shows that in patches of medium size, both males and females of poor
condition stay more in the edge zone than expected by random usage of the patch. The same
tendency can be seen in the only small patch with a stable population including animals of
both condition types. Thus, in these patches, animals were to some degree segregated with
respect to condition. Also, good-condition males moved more than poor-condition males.
Because I chose to study behaviour of several patches somewhat extensively rather
than one patch more intesively, I have no evidence that good-condition males have the highest
mating success. However, it would be highly surprising if this was not the case. High-quality
males sang about double as often as low-quality males; this alone should attract more females
(quiet males do not attract females at all).
10
Also, they sang much more loudly in the
Condition and spatial distribution
frequencies heard by humans, and probably also in the ultrasound frequencies. Song intensity
is generally strongly related to attractiveness (e.g., Farris et al. 1997 and references therein)
and can override other characteristics of the song (e.g., Snedden and Greenfield 1998).
Finally, Keuper et al. (1995) and Kalmring et al. (1990) suggested that male wart-biters move
so frequently during display because its song does not reach very far (the wart-biters song has
dominant frequencies of 12 kHz and a complex temporal structure, and is strongly reduced
and distorted in grassland and meadow). Thus, a male that moves more when singing can be
expected to attract more females. The range areas did not differ with coindition, however,
which indicates that good-condition males moved more back and forth. Finally, during the
song period, females of both condition stayed closer to the center than poor-condition males,
indicating that good-condition males indeed have higher mating success.
Although the data from this study are not suitable to reveal the proximate and
ultimate causes for the observed differences in space use, I find it interesting to discuss
possible causes of the observed pattern.
First, for males the most important proximate
mechanism is likely to be acoustic interactions between calling males. Keuper et al. (1986)
found that at long distances, wart-biter males are attracted by other males’ song, but at short
distances, the song has a repellent effect. When two singing males come close together, they
sing in in unison for a few minutes until one of them, or both, draw back (Keuper et al. 1986;
personal observation). In addition, Weidemann et al. (1990) found that within aggregations,
males had a tendency to be distributed with equal nearest-neighbor distances. Thus, males
appear to "defend" a circle using the song. In this study, poor-condition males may be driven
out in the periphery by asymmetric song interactions. Whether song frequency (e.g., when
one male sings but the other does not) or song amplitude is most important in this asymmetry
is not known. The mechanism behind the females’ patterns of habitat use is much less certain.
It may involve response to the males’ song. I have also observed physical aggression between
females, including once in this experiment when one female attacked and kicked another.
Since I did not observe the animals continuously, such agonistic encounters might have
happened regularely without my notice. In contrast I have never observed male-male physical
aggression.
There are at least two possible ultimate (or evolutionary) causes for space
partitioning. The first hypothesis is that the correlation between condition and patch position
is analogous to the situation commonly encountered in vertebrate leks, where the dominant
and successful males are centrally placed in the lek (e.g., black grouse ; Höglund and Alatalo
11
Paper III
1996:122-136). The motivation for this hypothesis is that the mating system of wart-biters at
least superficially appears to have many of the characteristics of leks. First, males put on a
display while clumping together in excess of what can be expected from habitat variation
(Weidemann et al. 1990). Second, there are ritualized contests between males. Third, there
seem to be no parental investment on part of the males (Wedell and Arak 1989). In contrast to
"classical" vertebrate leks (e.g., Bradbury 1985), males have no fixed territories, but sing for
short periods from each perch and move intermittently. Fixed territories, however, may be a
widespread consequence of lek systems rather than a crucial component of it.
E.g.,
"classically" lekking species such as black grouse (Tetrao tetrix) have mobile aggregations
similar to the wart-biters when they lek on ice-covered lakes, where there are no landmarks
(Hovi et al. 1996). However, much more information about the wart-biter mating system is
needed to confirm or reject that they lek. For instance, I have not demonstrated that the
central, high-quality males actually have greater mating success than the peripheral males.
The other hypothesis is that dominant males seek to minimize predation risk by
avoiding the patch edges. Increased predation and parasitation close to the habitat edge is
well-known from some bird communities (e.g., Brittingham and Temple 1983).
In
experimentally fragmented habitats, the adult and litter survival of root voles Microtus
oeconomus decreases sharply with increasing edge utilisation due to bird predation (Hovland
et al. 1999, Gundersen and Andreassen 2000). In concordance with the predation hypothesis,
subordinate individuals of both sexes (such as nonreproductive individuals and individuals
with low body mass) used the habitat edges more frequently than dominant individuals
(Hovland et al. 1999, Gundersen and Andreassen 2000).
As mentioned, we cannot only speculate around the question of which ultimate cause
that is most probable. None of the hypotheses is inconsistent with the belief that asymmetric
acoustic interactions are the proximate mechanism for the space partitioning between males.
In the case of the predation hypothesis, the high edge use of poor-condition males may simply
be a side effect of their instinct of keeping a distance to the dominant males. However, the
fact that male wart-biters clump also in non-fragmented habitats (Keuper et al. 1986) may
indicate that position relative to conspecifics is at least as important as position relative to the
patch geometry (i.e., avoiding the edges).
The finding that also females exhibit space partitioning, may be more consistent with
the predation hypothesis than the lek hypothesis. However, Pie (1988) observed contests
between females in the lekking fly Setellia sp., but they were less ritualized than male-male
12
Condition and spatial distribution
contests, which in most cases were presentations rather than physical fights. Females may
compete simply to have fast access to any male (which assumes that limited by access to fresh
sperm), or they may compete for the best males.
The former appears less likely; e.g.,
Reinhardt et al. (1999) found that although females of the grasshopper Chorthippus parallelus
mate every 4.5 days if they have access to males, the fertility of females that were allowed to
mate only once did not decline over a period of about a month. The latter In the wart-biter,
there is pronounced sexual selection on the postcopulatory level. Wedell (1991) found that
sperm competes numerically and not by last-male precedence, as is common in insects with
multiple-mating females (Davies 1991), and that the size of the spermatophylax influences
sperm competition by determining how much sperm that enters the female. There is therefore
ample
potential
for
Fisherian
selection
for
preferring
males
that
offers
large
spermatophylaxes. For instance, in the field cricket Gryllus bimaculatus, sons of males that
were chosen by females obtained double as many matings as sons of males that were not
chosen (Wedell and Tregenza 1998). The finding that females seem to move inwards form
the edges at mating time does not yield support to either of the hypotheses; they may be
attracted by the best singers, facilitating quick mate choice, or decreasing predation risk
during mating.
Why was there no space partitioning in the large patches? The cause may be the size
of the patch, or the size of the population, or a combination. The size of the patch is the most
plausible explanation. First, the proportion of interior areas is higher in larger patches, which
means less individuals per interior area. For males, this effect may be exaggerated by their
tendency to clump, at the same time as ranges did not increase from medium to large patches.
Thus, there may be more available space where poor-quality males can use and still keep an
acceptable distance to high-quality males. However, population size per se influence the
mating success of high-ranking males relative to low-ranking males (Widemo and Owens
1995) as well as the females’ success in finding suitable males (Kokko 1997). Consequently,
behavioural strategies (including movement) may change with popualtion size. Of course, we
cannot tease the effects of population size and patch area in this study, since they in principle
are perfectly correlated.
Regardlessof the ultimate cause for space partitioning, the observed effect of
patch/population size on wart-biter behaviour is interesting in the light of the excessive
fragmentation of the wart-biter’s natural habitat. A few decades ago, this species was common
in Norwegian semi-natural, unfertilized meadows, commonly with areas of several thousand
13
Paper III
m2. At present, there are almost no such habitats left (due to fertilization, forest plantation,
etc.). The wart-biter still appears to be fairly common, but almost all populations are in
roadsides and other very small remnant habitats (usually <1000 m2; Hjermann and Ims 1996).
The present paper indicates that such a shift in average habitat area may have other and more
subtile behavioural effects in addition to the well-known effects of on genetic and population
dynamic processes. Such behavioural effects of habitat size and form may have unexpected
consequences on the viability of populations, e.g. by altering mate choice, mating skew and
the effective population size.
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16
Condition and spatial distribution
Table 1. Tests of effects of sex, rearing (lab-bred or adult-caught) and patch size on the use of edgezones, measured by the overall habitat use and by habitat selection within days. Single factors were
tested using one-way anova, while the tests for interaction effects are type III tests from two-way
anovas (tests of the main effects are not shown).
Overall habitat use
Selection within days
Effect
d.f.
F
P
d.f.
F
P
Sex
1,59
0.01
0.97
1,53
0.01
0.91
Sex x Rearing
1,57
0.03
0.87
1,51
0.25
0.62
Sex x Patch size
2,55
1.41
0.25
2,49
1.12
0.33
Patch size x Rearing
2,55
3.59
0.034
2, 49
3.84
0.028
Patch size (among lab-reared animals)
2,40
6.36
0.004
2, 37
6.68
0.003
Patch size (among animals caught as adults)
2,15
0.76
0.49
2, 12
0.97
0.41
17
Paper III
Fig. 2. Four representative examples (one for each "type" of animal) of animal’s movements within a
medium patch (4x8 m). Some dates are marked to give an impression of the temporal scale. The
border between the edge zone and the interior is marked with a broken line. The edge selection index
SIedge for each individual is note expected to be 0.5 when the edge zone and the interior is used equally
much. (Note that this index does not reflect the area use of the A5 female very well, parlty because
observations outside the patch is ignored.)
Fig. 3. Proportion of observations where males sang (on the left axis) and where females laid eggs
(right axis) during the day.
Fig. 4. Average edge selection index for poor- and good-condition animals on patches of three sizes
(1, 2 and 3 denote small, medium and large; the large patches were double the size of the medium).
The indicated standard errors (1SE) were calculated using each individual (not each wart-biter
observation) as one statistical observation.
Fig. 5. Each individual’s average distance to the patch center during the day, based on the medium
patches only. One standard error is indicated; standard errors were calculated using each wart-biter
observation as one statistical observation.
Fig. 6. Area of the wart-biter ranges, calculated as explained in the text. There were no significant
effect of neither sex nor the condition treatment, but a significant effect of patch area (F2,53 = 6.26,
P=0.004). The indicated standard errors were calculated using each individual (not each wart-biter
observation) as one statistical observation.
18
Condition and spatial distribution
F emales
R ange are a
Males
16
F emales
Males
Are a (sq. m )
12
8
4
0
0,5
S m all
1,5
M edium
2,5
Large
3,5
P a tch siz e
Fig. 6. Area of the wart-biter ranges, calculated as explained in the text. There were no significant effect
of neither sex nor the condition treatment, but a significant effect of patch area (F2,53 = 6.26, P=0.004). The
indicated standard errors were calculated using each individual (not each wart-biter observation) as one
statistical observation.
Fig. 4. Average edge selection index for poor- and good-condition animals on patches of three sizes
(1, 2 and 3 denote small, medium and large; the large patches were double the size of the medium).
The indicated standard errors (1SE) were calculated using each individual (not each wart-biter
observation) as one statistical observation.
19
Paper III
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Fig. 5. Each individual’s average distance to the patch center during the day, based on the medium
patches only. One standard error is indicated; standard errors were calculated using each wart-biter
observation as one statistical observation.
20